The present invention relates to a deproteinized natural rubber latex, which is superior in balance between the film forming properties by means of the anode coagulation method and the dispersion stability of a latex, a method of preparing the same, a rubber product using the deproteinized natural rubber latex, and a proteolytic agent for natural rubber latex.
Natural rubbers have widely been used in various fields, for example, rubber gloves because of features such as large extension, high elasticity and strong film strength.
In the production of a glove made of a natural rubber, a production method is employed according to the thickness of a rubber film. A rubber glove having a film thickness of about 1 mm, for example, glove for home use is generally produced by a so-called anode coagulation method of dipping a mold (hand mold) for glove, the surface of which is previously coated with a coagulant (anode coagulant), in a natural rubber latex.
It has recently been required for a rubber product using a natural rubber latex to highly remove a protein contained in the product. Main reasons include (1) immediate (I type) allergy such as dyspnea or urticaria is caused by bringing a natural rubber product into contact with the skin or mucosa and a protein contained in a natural rubber latex is considered to be a causative agent; (2) the protein can cause variations in quality and vulcanization properties of the natural rubber product because the kind and quantity of the protein vary depending on the locality and production season of the latex; and (3) the protein can cause deterioration of mechanical characteristics such as creep characteristics and aging resistance and electrical characteristics such as insulating properties of the rubber product.
Japanese Published Unexamined Patent (Kokai Tokkyo Koho Hei) No. 6-56902 discloses a method of removing a protein and a decomposition product thereof through a series of the steps of adding a proteolytic enzyme (protease) and a surfactant to a natural rubber latex, maturing the natural rubber latex, thereby decomposing a protein in the latex, and subjecting the latex to a centrifugation treatment. When subjected to a deproteinization treatment according to this method, the protein in the natural rubber latex can be removed in a very high level and the nitrogen content (N %) as measured by the Kjeldahl method is reduced to 0.1% by weight or less.
For the purpose of preventing coagulation of a rubber component caused by an operation such as stirring by stabilizing a latex unstabilized as a result of removal of a protein, a surfactant is incorporated into a so-called deproteinized natural rubber latex obtained by the method disclosed in the publication described above.
The surfactant not only improves the mechanical stability of the latex, but also exerts a large influence on the sensitivity to an anode coagulant. When using a higher alcohol sulfate ester salt anionic surfactant as the surfactant, the sensitivity of the latex to the anode coagulant increases. Therefore, a film can be formed by the anode coagulation method even under the same conditions as those in case of the non-deproteinized natural rubber latex.
However, because of too large sensitivity to the anode coagulant, the film is rapidly dried after dipping in the latex as compared with the case of using the natural rubber latex. As a result, when a mold is repeatedly dipped in the latex for the purpose of increasing the thickness of rubber film, there arise new problems such as uneven thickness of the rubber and liquid dripping.
When using a higher alkyl phenyl ether sulfate ester salt anionic surfactant as the surfactant, the sensitivity of the latex to the anode coagulant decreases and the rubber component is less likely to be coagulated. To form a film having nearly the same thickness as that in case of the non-deproteinized natural rubber by the anode coagulation method, there arise new problems that very special film forming conditions or a very complicated step are required.
The present applicants have previously found such a fact that a dip product having a sufficient film thickness can be obtained by using a specific heat sensitizer and a specific anode coagulant in a specific combination and incorporating the specific combination in a large amount as compared with a conventional formulation (Japanese Published Unexamined Patent (Kokai Tokkyo Koho) No. 2000-17002).
However, there were problems, according to the method described in the publication described above, since the both of the heat sensitizer and the anode coagulant are incorporated into the latex, the latex becomes unstable as compared with a conventional heat sensitizing method using a natural rubber latex, thereby making it impossible to obtain long-term stability and making it hard to control heat-sensitive properties.